Ball ammunition uses bullets that are solid or non-expanding and may include musket balls, lead bullets and metal jacketed (such as full metal jacket or “FMJ”) bullets. Ball ammunition is typically used by military forces.
Hollow-point ammunition uses bullets that are designed to expand when they hit a target and thereby provide a larger diameter permanent cavity in the target, as well as providing a larger temporary cavity. Hollow-point ammunition is typically used by law enforcement and for personal defense. Hollow point bullets have a hollowed cup at the forward end of the bullet and are designed so that expansion or mushrooming of the bullet's hollowed cup occurs upon or after impact, increasing the effective diameter of the bullet. As the bullet moves through a target at a high rate of speed, material is picked up by the cup and compressed. When the outward pressure created by the material being compression inside the cup exceeds the yield strength of the cup wall(s), the cup mushrooms outwardly, increasing the effective diameter of the bullet. The faster the bullet traverses the medium, the greater is the compression of the captured material. However, the mushrooming of hollow point bullets is dependent on the bullet's terminal velocity; materials such as clothing fabric may become trapped in the cup and block other materials so that mushrooming does not occur. Hollow points seem not to expand reliably or at all in short barreled pistols that are carried for personal defense. Thus, the performance of hollow point bullets is considered to be unreliable in such weapons.
When bullets enter a human target or a ballistic gelatin target there is created a permanent cavity called the crush cavity. There is also created a temporary cavity known as the stretch cavity. The diameter of the crush cavity can be smaller than the caliber of the bullet that made the cavity. Gelatin, like human tissue, will part to admit entry of the bullet and will tend to close behind the bullet after entry and the bullet proceeds along its path. The diameter of the stretch cavity, unlike that of the crush cavity, is usually substantially larger than the diameter of the bullet. This temporary cavity is created by the turbulence of the bullet as it cuts its path through gelatin or tissue. The shape and size of the stretch cavity is defined by the shock and pressure wave associated with a particular bullet shape. Ball and round nosed bullets create the least turbulence. Hollow point bullets, with their cupped front ends, create more turbulence than ball and round nosed bullets whether they expand or not. Typically, injured tissues in the crush cavity are permanently damaged, while tissues in the stretch cavity suffer only temporary damage.
The Federal Bureau of Investigation's “Handgun Wounding Factors and Effectiveness” (1989) discusses the issues facing law enforcement officers who encounter an armed assailant. The article notes that incapacitation of a human target by gunshot wounds is highly unpredictable. Only a shot to the brain or upper spinal cord is considered to have a reliably predictable immediate outcome. Shots in the heart or circulatory system leading to massive bleeding are considered to take time that might allow the assailant an opportunity to injure the officer. Physiological damage from shots placed elsewhere were considered to be unlikely to have any immediate effect. Specifically, the article argues that physiological reaction is typically not the controlling factor in determining incapacitation; rather, it is the psychological reaction of the human target. Awareness of the injury; fear of injury, pain, and death; and loss of desire to continue in an attack all lead to cessation of conflict and to incapacitation of the assailant. However, in many cases, the human target may not even be aware of the wound. Emotional reactions such as rage can block awareness of damage. The effects of adrenaline, stimulants, narcotics, or pain killers may all block awareness of the injury. In such cases, the assailant will continue an attack despite being wounded. There are numerous accounts of shootings in which individuals received lethal gunshot wounds without an immediate cessation of the activity in which they were engaged. It is not unusual for shooting victims to fail to notice they have been shot.
The FBI article concludes that apart from the nervous system shots, the most reliable outcomes requires use of larger diameter bullets that penetrate 12-18 inches into a human target. However, other studies have suggested there may be other factors at play. Evan P. Marshall's and Edwin J. Sanow's 1992 book “Handgun Stopping Power” and follow-up 1996 book “Street Stoppers” (both published by Paladin Press) attempted to document the “stopping power” of various calibers and bullet configurations based on actual recorded shootings. Sanow/Marshall gave Winchester 32 caliber 60 grain Silvertips a 63% OSS (“One Shot Stop”) effectiveness rating, which was surprising because they gave the same rating to both .45 caliber 230 grain Federal and .45 caliber 230 grain Winchester full metal jacket ball ammunition. The .32 caliber Silvertips achieved a mere muzzle energy of 125 foot pounds, versus the 356 foot pounds that the .45 Winchester and .45 Federal rounds register. Many of the Sanow/Marshall conclusions have been challenged by authors such as Duncan MacPherson and Martin L. Fackler, M.D., and others, who have contended that bullet weight, size and speed are the only relevant factors in a stopping power analysis.
The argument over the relative stopping power of various bullet shapes, weights, sizes and velocities is likely never to be fully resolved since experiments with living targets is not possible or acceptable. However, experiments with ballistic gelatin and field reports of actual shootings do offer strong clues as to a bullet's stopping power efficacy. The evidence suggests both the crush cavity and stretch cavity play an important role in stopping power. The evidence also strongly suggests that hollow point bullets seem to be most effective when they expand, but that expansion is at best unreliable and inconsistent. That said, even expansion is not a true indicator of a bullet's stopping power potential, since a hollow point bullet's expansion relies on the yield strength of the hollow point cup. A bullet made of cheese would expand enormously but would lack penetration. Cup expansion is as much a measure of the yield strength of the cup as it is of potential stopping power. Expansion has to be coupled with adequate penetration.